Method of treatment of liquid steel under vacuum

ABSTRACT

A METHOD OF DE-GASIFICATION OF STEEL IN FUSION POURED INTO A CASTING LADLE IN WHICH ARE IMMERSED THE LOWER EXTREMITIES OF TWO VERTICAL REFRACTORY PIPES, THE UPPER PARTS OF WHICH OPEN INTO THE BOTTOM OF A VACUUM-TIGHT REFRACTORY CHAMBER, THE ONE IN THE PROXIMITY OF ITS CENTER AND THE OTHER INTO ITS PERIPHERAL PORTION, A CONSTANT VACUUM ENSURING THE RISE OF THE STEEL BATH AND ITS STABILIZATION AT A PRE-DETERMINED HEIGHT BEING MAINTAINED IN SAID CHAMBER, THE STEEL BEING HEATED BY INDUCTION, A CIRCULATION OF THE LIQUID STEEL THROUGH THE PIPES BETWEEN THE LADLE AND THE CHAMBER BEING EFFECTED BY MEANS OF AN INDUCTOR WINDING SURROUNDING THE CHAMBER AND FOR A PERIOD SUFFICIENT FOR THE DESIRED DEGREE OF PURIFICATION UNDER VACUUM TO BE OBTAINED.

June 5, 1973 .1. POMEY 3,737302 METHOD OF TREATMENT OF' LIQUID STEEL UNDER VACUUM Filed July 15, 196e United States Patent O 3,737,302 METHOD OF TREATMENT OF LIQUID STEEL UNDER VACUUM Jacques Pomey, Billancourt, France, assignor to Societe des Aciers Fins de lEst, Billancourt, France Filed July 1s, 196s, ser. No. 744,744 Claims priority, application France, July Z7, 1967,

5 94 Int. Cl. (121e 5/52; C22d 7/00; H051) 5/12 U.S. Cl. 75-10 9 Claims ABSTRACT OF THE DISCLOSURE A method of de-gasiiication of steel in fusion poured into a casting ladle in which are immersed the lower extremities of two vertical refractory pipes, the upper parts of which open into the bottom of a vacuum-tight refractory chamber, the one in the proximity of its center and the other into its peripheral portion, a constant vacuum ensuring the rise of the steel bath and its stabilization at a pre-determined height being maintained in said chamber, the steel being heated by induction, a circulation of the liquid steel through the pipes between the ladle and the chamber being eiected by means of an inductor winding surrounding the chamber and forV a period sufficient for the desired degree of purification under vacuum to be obtained.

It is already known to employ a vacuum in order to obtain the liberation of hydrogen which liquid steel, alloys and other metals may contain and also to eliminate oxygen in the form of carbon monoxide. The combination of these well-known reactions is produced for example in the preparation of steel under vacuum in an induction furnace. It is also known that steel prepared by any process involving the use of a Martin furnace, a Thomas converter, an Heroult electric furnace, etc., can b'e poured into a steel-works ladle and treated by vacuum in the ladle before being poured into anfingot mould.

The present invention concerns this latter process. It is known that one of the disadvantages of theknown methods of treatment of steel in a steel-works ladle is that neither the temperature nor the time of treatment sists of enclosing the ladle of steel in a chamber in which a vacuum is created. This method has the particular disadvantage of necessitating a large volume which correspondingly increases the time necessary to obtain the vacuum, due to the necessarily limited capacity of the pumping equipment. In addition, it is desirable to ensure stirring of the steel. l

This can be effected by bubbling argon which is introduced through a porous brick mounted at the bottom of the ladle. This has a two-fold disadvantage; on the one hand, the brick must be changed and suitably sealed for each pouring, and on the other hand, in addition to the vacuum, it is necessary to utilize argon. The invention also enables all these complications to be avoided.

IIn another also known method of treatment of steel in which the ladle is placed in a vacuum chamber, use is made of electro-magnetic stirring induced by an inductor winding placed in the vacuum chamber, at its lower portion round the ladle. This method makes it necessary, on the one hand to employ a frequency of the order of 0.5 to 2 Hz. which does not permit any heating by the Joule er'ect and on the other hand, it necessitates that the ladle should have a casing of nonmagnetic steel (stainless austenitic, 18 Cr-8 Ni) sozas-to avoid it becoming the seat of large induced currents. These two compli- ICC cations can be avoided by means of the present invention. In other cases, to which the new method according to the invention can be applied, the vacuum is created in a luid-tight refractory chamber having at its lower portion one or two pipes immersed in the liquid steel contained in a conventional steel-works ladle. As a result of the vacuum, the steel rises from the ladle into this chamber to a level defined by the hydrostatic pressure. In this way,

the steel which is in this chamber is subjected to a vacuum. In these methods, as in previous methods, the operator cannot freely control either the temperature or the time. In the method according to the invention, on the contrary, he is freed from these two limitations.

In certain known methods, the chamber is preheated by a cylindrical graphite rod which passes right through the chamber and which is heated by the Joule effect of a low-tension electric current. In consequence of the insuliiciency of radiating surface, this method of pre-heating does not permit the chamber to be maintained at a temperature equal to or higher than that of the steel; it cannot wholly compensate for the heat losses and a fortiori, it cannot heat the steel. The method according to the invention enables all these advantages to be overcome.

As during the course of the operation, it is necessary that the whole mass of steel pass through the vacuum chamber, a circulation or a stirring action is necessary. In certain known methods, this is obtained by carrying out a relative vertical alternating movement between the chamber and the ladle, in such manner that, taking account of the hydrostatic level which is higher in the vacuum chamber than in the ladle, the steel passes in and out of the chamber at each oscillation, thus causing a renewal of the steel in the chamber. This method has two disadvantages: on the one hand it is necessary to subject very heavy equipment to these vertical oscillations: either the vacuum chamber or the ladle full of steel, and on the other hand, the free level of the steel, alloy or other metal is also subjected to vertical oscillations, which results in a cooling and a continual re-oxidation by atmospheric air, even if the steel bath is covered with slag. In the method according to the invention, these two disadvantages are avoided.

' In other known methods, the vacuum chamber is connected to the ladle by two plunging pipes and a circu latory flow of the steel is established, rising inside one pipe and falling inside the other. It is from this principle that the method according to the invention is derived.

In certain known arrangements, this flow is established by bubbling argon through one of the two pipes, which reduces the mean density in this column, so that the steel in the column circulates from the bottom towards the top. This method makes it necessary to utilize argon, and further has the disadvantage of reducing the degree of vacuum and of being an additional cause of cooling. These disadvantages are avoided by means of the method according to the invention.

In other known arrangements, it has been sought to create a circulation by an electro-magnetic eiect produced at the level of these pipes, but the effectiveness of this is wholly insuilicient. On the other hand, the invention permits an intense and very eiective circulation to be obtained.

Finally, attempts have been made to heat the steel in the plunging pipe by means of an inductor winding, but the eifect is definitely insuicient in consequence of the small section of the pipe. On the contrary, it is possible to obtain eiiicient heating by means of the method accordv ing 'to the invention.

The new method enables all the disadvantages shown by known methods to be remedied, while at the same time all their advantages are retained. As indicated above,

it starts from the method of de-gasication of the molten steel, during the course of which the liquid steel is poured into a casting ladle and covered with a layer of slag. The lower extremities of two refractory vertical pipes are then immersed in the casting ladle, the upper portions of these two pipes passing into the bottom of a fluid-tight refractory heat-insulated chamber, the form Of which is that of a body of revolution with a vertical axis.

The gas contained in the chamber is sucked out so as to maintain a constant vacuum, ensuring the rise of the steel bath and its stabilization at a pre-determined hydrostatic height, a circulation of liquid steel 'being effected through the intermediary of the pipes between the ladle and the chamber, while the steel is heated by induction.

The method according to the invention is characterized by the fact that the said circulation is induced by means of an inductor winding surrounding the refractory charnber insidel an outer fluid-tight metal casing and traversed by an alternating current, the voltage and the frequency of which, are so chosen that the current induced in the liquid steel produces a centripetal radial electro-magnetic force, which tends to move the steel from the peripheral part of the chamber towards its centre. One of the two pipes opens in proximity to the axis of the chamber; the other opens into the said peripheral portion.' The winding is cooled by water circulation as in the case of all induction furnaces working at high or medium frequencies. The circulation, without modification of the level of the steel in the chamber, is maintained for a sufficient period to obtain the degree of purification desired throughout the whole mass of the steel, following which the vacuum is removed in the chamber by the introduction of a neutral or reducing gas after each degasifying operation.

The voltage and the frequency are advantageously chosen such that the current induced in the liquid steel produces, in addition to the said radial force, a Joule effect which ensures the heating iof the liquid steel in the chamber.

The refractory chamber may be heated internally before each degasifying operation and during this operation 'by means of a horizontal and coaxial graphite ring housed in the upper internal portion of the refractory chamber and heated by induction by means of another inductor winding surrounding the chamber and inside the external metal casing.

The invention also extends to certain details of application of the method together with the devices permitting this method to be carried into effect, these details and devices being brought out in the description which follows below, given by way of example, and in which reference will be made to the accompanying diagrammatic drawings, in which:

FIG. l is a very simplified view in vertical section of a device for carrying the method into elect;

FIG. 2 is a more detailed view in cross-section of the same device taken along line II-II of FIG. 3; and

FIG. 3 is a view in horizontal section taken along the line III-III of FIG. 2.

In its general lines, the refractory vacuum chamber .1, shown in diagrammatic form in FIG. l, is a vertical cylinder of revolution closed at its base along a straight horizontal section 2 and at its upper portion by a spherical cap or dome 3. The vacuum connection 4 is a pipe placed for example in the dome 3. The base comprises two vertical pipes plunging into the steel bath contained in the ladle 9. One of these pipes 5 is located at X-X' approximately in the axis Z-Z of the vacuum chamber while the other pipe 6 is located laterally at the periphery against the cylindrical wall of the vacuum chamber, along the axis Y-Y.

When the vacuum is created in the chamber 1, the steel rises in this chamber to a height H above ,the free surface in the ladle, this height being defined by the hydrostatic equilibrium:

in which p1 is the atmospheric pressure, p0 is the pressure inside the vacuum chamber, and is the density of the liquid steel, alloy or other metal. For this reason, the steel rises in the chamber 1 to a level h above the base 2. An inductor winding 7 with water circulation surrounds the whole of the refractory body and the heat insulation of the cylindrical chamber, as in the case of coreless induction furnaces. This winding is traversed .by a single phase alternating inductor current of appropriate frequency. This frequency may be that of the commercial electric power or a lower frequency, and a capacitor in parallel can keep the voltage and current in phase.

This arrangement results in induced currents in the bath of steel 8 which have two effects:

l) A centripetal electro-magnetic force which ensures the circulation of the steel from the periphery towards the axis, from which it again iiows down into the ladle, while a fresh quantity of steel coming from the ladle rises inside the lateral pipe and reaches the .vacuum chamber;

(2) A Joule effect which compensates for the heat losses of all kinds.

The ratio of the effective power Weff to the pressure P generated in the axis -by the centripetal electromagnetic force is:

Wen 12d l p in which l is the height of the inductor winding and h is the height of the lbath of steel in the vacuum chamber.

e is the conventional thickness of the skin-effect currents and p is the resistivity of the steel.

By construction, d is given a high value, for example 2 m. for an installation treating a ladle of 40 tons of steel so as to permit -a good action of the vacuum.

It can be seen that for a given electric power Weff, the pressure P which causes the circulation of the steel increases inversely as the square root of the frequency f, which shows the advantage of a low frequency, for

example that of the commercial electric power (50 Hz.

in France) or a lower frequency which is however higher than or equal to 15 Hz. so as to produce simultaneously the circulation andV the heating of the steel. By construction, the height of the inductor winding l is given, this being of the same order as the height h of the bath of steel in the vacuum chamber; there is therefore an advantage in giving l a fairly low value so as to give a high ratio of P/ Werf. The value of l being thus fixed by construction, the ratio P/ Weff can be regulated at that moment by the level h of the steel in the vacuum chamber, which, for a given degree of vacuum, is obtained by the adjustment ofthe position of a jack lwhich actuates a lifting device on whichv the ladle rests.

In actual fact, the structure of the apparatus is rather more complicated. In fact, in order that the circulation I`between the steel and the ladle may be a maximum, it is necessary (see FIG. 2) that the-lateral delivery pipe of the steel coming from the ladle 9 delivers into the base 2 of the vacuum chamber through a peripheral channel 10 covering a sufficient-circular arc, but the Width of which counted radially is less than the thickness e of the skinl currents. Thus, fora chamber having a diameter d=2 m. and an inductor current of 50 Hz., the radial width e of this channel is preferably comprised between 8 and 12 cm., while its depth is determined at will and is preferably decreasing aS it moves away 0n each side from' the axis Y-Y of the lateral pipe. This circular varc may be as large as desired and may even cover the complete'circumference in order thus to ensure the best centripetal distribution of the steel in the vacuum chamber and thereby to ensure the best efliciency of the vacuum. In this case', the radial width e of the channel is constant, but its depth decreases up to the point P, diametrically opposite to the axis -Y-Y of the lateral pipe.

In addition, instead of being located strictly in the axis of the chamber, the down-pipe 5 is slightly displaced towards the side opposite to the rising pipe, for the purpose of improving the uniformity of the centripetal circulation of the steel in all directions.

The vacuum-tightness is ensured by a metal casing 11 external to the refractory chamber 22, to its heat lagging 23 and to the inductor winding 7. In order to do this, following the standard technique, the lines of magnetic flux are channelled to the exterior of the refractory chamber and lagging, to the exterior of the inductor winding and to the interior of the uid-tight casing by packets of iron sheets 12 or of silicon iron suitably cut-out and oriented in meridian planes.

The upper part of the chamber contains the heating device. This device (see FIG. 2) consists of a coaxial graphite ring 13 located inside the heat-insulated chamber and heated by induction by means of a winding 14, cooled by water and placed outside the refractory heat-insulated chamber.

The ring 13 may be constituted by a sludge of graphite agglomerated while hot with molten pitch, or better still it may be an assembly of graphite bricks put together dry or with a hot illing constituted by graphite powder and molten pitch. In both cases, the binder is a conductor of electricity and its pyrogenatiou converts it to monolytic graphite which is a very good conductor of electricity.

The frequency of the single-phase alternatingl inductor current is chosen between 150 and 1500 Hz., taking into account the thickness of the ring and its resistivity.

For installations of large capacity, this current may advantageously be a current having a frequency three times or better still nine times that of the commercial supply, by means of static frequency multipliers known per se. Thus, in an apparatus of 2 m. diameter for a ladle of 40 tons of steel, the advantageous frequency is 450 or 540 Hz., depending on whether the commercial supplyfrequency is 50 or 60 Hz. In this case also, the magnetic eld is channelled between the metal vacuumtight casing 11 and the winding 14 by packets of iron sheets 15, appropriately cut-out and placed in meridian planes. The empty spaces between the packets may advantageously be packed with a suitable ferrite, a substance with the formula FezOaMO in which M is a suitable bivalent metal; these are well known insulating and ferro-magnetic substances.

The utilization of the above device -is as follows:

Diaphragms of very thin steel sheet are stuck by means of a suitable glue, such as Araldite, to the lower extremities of the pipes 5 and 6, thus closing the two pipes. The vacuum is created in the chamber; the current at 450 Hz. in the upper inductor heats the graphite ring 13 which, by conductivity and especially 4by radiation, progressively heats the whole chamber 1 to a temperature of 1600 C. Inst before the ladle arrives in position for treatment, the vacuum is destroyed by the introduction of nitrogen coming from a cylinder of compressed gas, or of a reducing gas such as blast-furnace gas.

In a melting plant of any kind, such as a Martin furnace, a Thomas converter, or an IIeroult electric furnace, amongst others, the steel was prepared without superheat, so that its oxidation condition was a minimum. It was poured into a standard steel-works ladle 9 which is for example a ladle lined with silico-aluminous refractory materials in a packed sludge or with assembled bricks, with a conventional metal frame of ordinary steel, thus ferro-magnetic and economical. This may be, according to the usual practice, a ladle with a gas-nozzle, a plug and a stopper-rod. The central metal rod of the stopper may be tubular and may be traversed by a ow of cooling air (not shown on the drawings).

The slag can be poured into the ladle with the steel alternatively the steel alone may be poured and can be covered with a thin layer of highly meltable synthetic eutectic slag and with a thicker layer 16 of the same slag CTI remaining in powdered form and heat insulating, and made reducing by powdered carbon, which ensures between the grains an atmosphere containing carbon monoxide and free from oxygen.

The ladle full of steel 9 is brought under the vacuum chamber 1 and a jack 17 lifts it vertically, so that the two pipes of the chamber plunge into the steel. The thin sheets melt and the steel passes into the two pipes. The vacuum is gradually restored, the steel passes into the chamber 1 and the current is connected to the lower inductor, which effects at vthe same time the heating of the steel and the centripetal circulation of the steel in the chamber. The steel rises up the lateral pipe 6 and falls down the axial pipe 5. In the ladle, the convection currents continually renew the metal which passes into the lateral pipe 6.

In the vacuum chamber 1, an intense bubbling takes place, due to the liberation of hydrogen and carbon monoxide; the first ceases due to exhaustion, the second continues as long as all the dissolved oxide and the emulsioned inclusions have not been reduced.

As long as the depression in the vacuum chamber remains constant, the level in the steel-works ladle also remains constant. The surface of the steel bath in the ladle is quiet. The steel is only subjected to horizontal movements resulting from convection. The powdered layer 16 which covers the slag is calm and carries out its heat insulating and de-oxidizing action.

The stopper rod is immersed in steel, in which slow convection currents take place, practically without turbulence so that if its axial down-pipe is cooled by a circulation of air, its operation is reliable and without risk of trouble as long as may be desired. The materials of the gas-nozzle and the plug may be chosen sufficiently refractory so that there is no risk of sticking. When the reducing action of the carbon continues during the course of treatment, the bubbling gradually slows down in the vacuum chamber with the gradual elimination of the oxygen. The degree of Vacuum in the chamber 1 increases, the height h increases and the height H is maintained at the desired value by acting on the jack which regulates the height of the ladle with respect to the chamber. This adjustment is made while noting that if H increases, the ratio heating/stirring for equal power is increased, and vice-Versa. There are therefore two adjustment parameters: electric power and the height H of the bath, which gives the method great flexibility of regulation which does not exist in the previously known methods.

Then, when the bubbling has practically ceased, and the degree of vacuum reaches its maximum value, the reducing action of the carbon takes place at the actual surface of the bath of steel, and this additional reduction is appreciable due to the intense circulation of the steel at the free surface in the vacuum chamber, this circulation being much more intense than in any of the other known methods. For this reason, the de-oxidation can be carried very far without being interfered with either by a hydrostatic pressure or by an interfacial tension or by a limited time.

The apparatus comprises one or more hoppers 18 comgrised between two gates 19 and 20 forming a lock cham- By opening the gate or the vacuum-tight cover 20, there is introduced into the hopper the dosed quantity of any additional constituent to be introduced into the steel. After closing the gate 20, the valve 21 is opened and puts the hopper 18 into communication with the vacuum chamber 1. At the desired moment, the gate 19 is opened s0 as to allow the additive to fall into the bath of steel contained in the ladle. By virtue of the heating system, the quantity of additives is not limited; by virtue of the previous de-oxidation of the steel and due to the vacuum, even highly oxidizable additives such as silicon, titanium, aluminum amongst others can be employed without direct oxidation by air and with very good eiciency.

Thus, after having proceeded to the final additions and waited until they are uniformly distributed throughout the steel, the vacuum is removed, preferably by filling the chamber 1 with an inert gas such as nitrogen, or a reducing gas such as blast-furnace gas. The ladle 9 is lowered and thus liberated is transferred to the area for pouring into ingot moulds.

The operation of treatment under vacuum can therefore be carried out for the time required for the total purification of the steel and in order that, at the end of this operation, the temperature is absolutely correct for subsequent pouing into ingot moulds, either by conventional pouring or by continuous pouring methods.

It is of course advantageous for this subsequent pouring to be carried out under complete protection from reoxidization by air, for example by means of immersed pipes, as described in French Pats. No. 1,464,005 of Nov. 15, 1965, by the present applicant for Method and Plant for the Continuous Casting of Steel, and No. 1,499,333

" of Sept. 9, 1966, by the Acieries du Temple, for Device for Pouring Steel Into Ingot Moulds.

In an alternative form of application of the illustrated devices the winding surrounding the lower part of the chamber is no longer required to create h'eating by the Joule effect, it is only required to ensure intense stirring by the electro-magnetic effect of the currents induced in the liquid steel contained in the chamber this steel becoming the seat of a centripetal electro-magnetic force which ensures the circulation of the steel between the ladle and the vacuum chamber.

The steel rises from the ladle into the vacuum chamber through the lateral pipe 6 and moves down from this chamber into the ladle through the axial pipe 5. As in this case it is only required that the induced electric currents should create an electro-magnetic effect which is desired to be as intense as possible, the frequency is very low, and is comprised between 1 Hz. and 20 Hz. In order to avoid induced currents in the casing 11, a ferromagnetic circuit of packets of iron sheets 12V is arranged between the winding 7 and the casing 11.

Thge vacuum chamber 1 can advantageously be equipped with the heating device comprising the graphite ring 13 heated by means of the winding 14, as has already been shown in FIG. 2.

The method according to the invention permits the production of steel, alloys, and other metal which are better de-oxidized than with the methods previously known, due to the fact that the operation can be prolonged for the time required and because of the methodical and intense circulation of the steel in the vacuum chamber. This method makes it possible to avoid excess heating and overoxidation of the steel at the time of pouring from the preparation furnace into the steel-works ladle; finally this method makes it possible to obtain the steel at the correct fairly-low temperature for de-oxidation under vacuum and also at the correct and higher temperature for pouring into ingot moulds, advantages which the previously known methods do not possess. l

The method according to the invention and the devices described for carrying it into effect have furthermore the following advantages:

(l) The construction of the device is simple and robust, due to the fact that there is no relative movement between the vacuum chamber and the ladle during the course of the treatment.

(2) The ladle is of the standard type with an external casing of ordinary ferro-magnetic steel.

' (3) The slag which covers the free surface of the bath of steel in the ladle is at rest, and for this reason provides excellent protection against heat losses or 4against oxidation by the atmosphere.

(4) The circulation of the steel inthe vacuum chamber is uniform and can be regulated by electrical means.

(5) The steel coming from the steel-works furnace is poured into the ladle at its normal pouring temperature without any overheating having been effected, and for 8 this reason the steel has a minimum amount of dissolved oxide. Y

(6) The duration of the operation of treatment under vacuum is regulated at will without any limitation resulting from the temperature, since the latter is maintained or increased at will by electrical regulation means.

(7) For this reason, de-oxidation by carbon can be carried as far as may be desired, and can thus result in steels of better quality than by the previously known methods.

. The method further provides:

(a) A powerful stirring effect regulatable by the intensity of the inducing current at very low frequency (10` Hz. for example);

(b) Pre-heating by induction at 450l Hz.

I claim:

1. A method of degasifying molten steel comprising pouring molten steel into a casting ladle, covering the molten steel with a layer of slag, im-ersing in said molten steel the lower extremities of two substantially vertical refractory pipes, the upper parts of which open into the bottom of a vacuum-tight heat-insulated refractory charnber which is in the form of a body of revolution with a vertical axis, maintaining a substantially constant vacuum in said chamber by which occluded gases are entrained by suction and by which molten steel is caused to rise into the chamber `and is lstabilized at a predetermined hydrostatic height, induction heating the molten steel, and circul-ating the molten steel between the ladle and the chamber through the two refractory pipes by passing an alternating current through an inductor winding which surrounds the refractory chamber inside an external metal casing, said alternating current inducing in the molten steel a radially centripetal electromagnetic force tending to move the molten steel from the periphery of the chamber towards the center of the chamber, the molten steel circulating downward through one of said pipes which opens into the chamber at about the center of the bottom of said chamber, and the molten steel circulating upward through the second of said pipes which opens at the periphery of said chamber, cooling the winding by passing a cooling uid in close proximity thereto, maintaining the circulation of molten steel at a substantially constant level in the chamber for a period sufficient to degasify the whole mass of molten steel t0 the desired degree of purification, and at the conclusion of the degasifying operation introducing a neutral or reducing gas into the chamber to remove the vacuum from said chamber.

2. A method in accordance with claim 1, wherein the voltage and the frequency of said alternating current are so chosen that the current induced in the liquid steel p-roduces, in addition to the said radial force, a Joule effect insuring the heating of the liquid steel in the said cham- 3. A method in accordance with claim 2, wherein the frequency is between 15 and 60 Hz.

4. A method in accordance with claim 2 further comprising the step of, during the de-gasifying operation, passing the steel into the chamber through the peripheral pipe which opens into the bottom of a peripheral channel formed in the base of the said chamber and covering a circular arc of at leas-t 180, the radial width of the said channel being less than the thickness of the induced `skin effect currents.

5. A method in accordance with claim 1, further comprising the `step of heating the refractory chamber internally before each de-gasifying oper-ation and during the operation, by a horizontal and coaxial ring of graphitel housed in the internal upper portion of the refractory chamber heated by induction by a second inductor winding'surrounding the chamber and inside the external metal casing.

6. A method in accordance with claim 5, wherein the frequency of the current passing through the said second inductor winding is between and 1500 Hz.

7. A method in accordance with claim 5, wherein the frequency of the current supply to each of the windings surrounding the chamber is between 1 and ZO Hz.

8. A method in accordance with claim 1 further comprising the step of covering the surface of the liquid steel poured into the ladle, which is covered with a layer of reducing molten slag, with a layer of reducing powdered material containing carbon.

9. A method in accordance with claim 1 further comprising the step of regulating the level of the steel in the chamber during ythe course of the de-gasifying operation by varying the height of the ladle with respect to the chamber.

References Cited UNITED STATES PATENTS 3,468,525 9/1969 Kawa 75-49 10 Erdely 75-49 'Dama 13-26 Fontana 75-11 Matsuda 75--93 Pagonis 75-10 Lorenz 75-10 Taylor 75-49 Mller 75-10 Floymayr 75-10 U.S. C1. X.R. 

